1. Field of the Invention
The present invention relates to a process for the preparation of 1,1,1-trifluoro-2,2-dichloroethane (CF.sub.3 CHCl.sub.2). More particularly, the present invention relates to an improved process for the preparation of 1,1,1-trifluoro-2,2-dichloroethane in which chlorination of 1,1,1-trifluoro-2-chloroethane (CF.sub.3 CH.sub.2 Cl) is carried out by feeding chlorine and hydrogen into a reactor simultaneously or sequentially in the presence of activated carbon, thereby enhancing the selectivity to 1,1,1-trifluoro-2,2-dichloroethane.
2. Description of the Prior Art
1,1,1-trifluoro-2,2-dichloroethane has similar physical properties to trifluorochloromethane (CF.sub.3 Cl, known commercially as "CFC-11"), but it hardly decomposes the ozone layer and shows little influence on the warning of the earth. For these reasons, 1,1,1-trifluoro-2,2-dichloroethane is attractive commercially as a potential substitute for CFC-11.
A number of processes for the preparation of 1,1,1-trifluoro-2,2-dichloroethane have been suggested. For example, European Patent Publication No. 0 346 612 A, and Japanese Patent Kokai (Laid-Open) Publication No. 89-290638, disclose a process for the preparation of 1,1,1-trifluoro-2,2-dichloroethane which comprises chlorinating 1,1,1-trifluoro-2-chloroethane in the presence of a catalyst such as CuCl.sub.2, NiCl.sub.2, and FeCl.sub.2 carried on AlF.sub.3. U.S. Pat. No. 4,060,469 discloses a process for the preparation of 1,1,1-trifluoro-2,2-dichloroethane which comprises photochlorinating 1,1,1-trifluoro-2-chloroethane under a radiation of specific wavelength range.
In general, a chlorination of 1,1,1-trifluoro-2-chloroethane is carried out stepwise in accordance with following reactions: EQU CF.sub.3 CH.sub.2 Cl+Cl.sub.2 .fwdarw.CF.sub.3 CHCl.sub.2 +HCl(1) EQU CF.sub.3 CHCl.sub.2 +Cl.sub.2 .fwdarw.CF.sub.3 CCl.sub.3 +HCl(2)
In the above reactions, Sequence (1) is the reaction of interest, and Sequence (2) is an undesired side reaction. However, it has been reported that the rate of Sequence (2) is faster than that of Sequence (1). Due to these characteristics, an amount of unnecessary by-product, CF.sub.3 CCl.sub.3, in addition to the desired chlorinated product, CF.sub.3 CHCl.sub.2, has been inevitably produced in the prior art processes. The by-product, CF.sub.3 CCl.sub.3 needs additional processes and apparatus for its treatment.
Attempts have been made to overcome the problems mentioned above by increasing the selectivity to CF.sub.3 CHCl.sub.2 as much as possible. A typical process is disclosed in U.S. Pat. No. 4,060,469 and Japanese Patent Kokai (Laid-Open) Publication No. 91-52831, in which chlorination is carried out at a low molar ratio of Cl.sub.2 to CF.sub.3 CH.sub.2 Cl so as to suppress production of CF.sub.3 CCl.sub.3. However, this process causes the problem that the conversion ratio of CF.sub.3 CH.sub.2 Cl is low, rendering the process economically ineffective.